# std::map<Key,T,Compare,Allocator>::emplace_hint

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< cpp‎ | container‎ | map

template< class... Args > iterator emplace_hint( const_iterator hint, Args&&... args ); |  | (since C++11)

Inserts a new element into the container as close as possible to the position just before hint.

The constructor of the element type (`value_type`, that is, std::pair<const Key, T>) is called with exactly the same arguments as supplied to the function, forwarded with std::forward<Args>(args)....

No iterators or references are invalidated.

### Parameters

hint | - | iterator to the position before which the new element will be inserted
args | - | arguments to forward to the constructor of the element

### Return value

An iterator to the inserted element, or to the element that prevented the insertion.

### Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

### Complexity

Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint.

### Example

Run this code

    #include <chrono>
    #include <cstddef>
    #include <functional>
    #include <iomanip>
    #include <iostream>
    #include <map>

    const int n_operations = 100'500'0;

    std::size_t map_emplace()
    {
        std::map<int, char> map;
        for (int i = 0; i < n_operations; ++i)
            map.emplace(i, 'a');
        return map.size();
    }

    std::size_t map_emplace_hint()
    {
        std::map<int, char> map;
        auto it = map.begin();
        for (int i = 0; i < n_operations; ++i)
        {
            map.emplace_hint(it, i, 'b');
            it = map.end();
        }
        return map.size();
    }

    std::size_t map_emplace_hint_wrong()
    {
        std::map<int, char> map;
        auto it = map.begin();
        for (int i = n_operations; i > 0; --i)
        {
            map.emplace_hint(it, i, 'c');
            it = map.end();
        }
        return map.size();
    }

    std::size_t map_emplace_hint_corrected()
    {
        std::map<int, char> map;
        auto it = map.begin();
        for (int i = n_operations; i > 0; --i)
        {
            map.emplace_hint(it, i, 'd');
            it = map.begin();
        }
        return map.size();
    }

    std::size_t map_emplace_hint_closest()
    {
        std::map<int, char> map;
        auto it = map.begin();
        for (int i = 0; i < n_operations; ++i)
            it = map.emplace_hint(it, i, 'e');
        return map.size();
    }

    double time_it(std::function<std::size_t()> map_test,
                   std::string what = "", double ratio = 0.0)
    {
        const auto start = std::chrono::system_clock::now();
        const std::size_t map_size = map_test();
        const auto stop = std::chrono::system_clock::now();
        std::chrono::duration<double, std::milli> time = stop - start;
        if (what.size() && map_size)
            std::cout << std::setw(8) << time << " for " << what << " (ratio: "
                      << (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n";
        return time.count();
    }

    int main()
    {
        std::cout << std::fixed << std::setprecision(2);
        time_it(map_emplace); // cache warmup
        const auto x = time_it(map_emplace, "plain emplace");
        time_it(map_emplace_hint, "emplace with correct hint", x);
        time_it(map_emplace_hint_wrong, "emplace with wrong hint", x);
        time_it(map_emplace_hint_corrected, "corrected emplace", x);
        time_it(map_emplace_hint_closest, "emplace using returned iterator", x);
    }

Possible output:

    365.08ms for plain emplace (ratio: 1.00)
     96.54ms for emplace with correct hint (ratio: 3.78)
    409.40ms for emplace with wrong hint (ratio: 0.89)
     85.57ms for corrected emplace (ratio: 4.27)
     84.31ms for emplace using returned iterator (ratio: 4.33)

### See also

emplace (C++11) | constructs element in-place (public member function)
insert | inserts elements or nodes(since C++17) (public member function)

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